Ockham's Razor

"Of two competing theories or explanations, all other things being equal, the simpler one is to be preferred."

...or if you would rather, in Latin: Entia non sunt multiplicanda praeter necessitatem ("Entities should not be multiplied beyond necessity.") This principle has been attributed to a 14th century logician and Franciscan friar, William of Ockham, and is also known as the Principle of Parsimony, the Principle of Simplicity, or even KISS ("Keep It Simple, Stupid.")

In terms of the scientific method, it can be used as a loose rule of thumb when examining competing scientific hypotheses; the one that contains the least possible number of unproven assumptions is the most likely to be fruitful.

In terms of engineering: quite simply "Near enough is good enough."

Other variants

Pluralitas non est ponenda sine neccesitate ("Plurality should not be posited without necessity.")

Often heard in medical schools: "When you hear hoofbeats, think horses not zebras."

Thargola's Sword: "We must drive a sword through any hypothesis that is not strictly necessary." (From Nightfall by Isaac Asimov and Robert Silverberg.)

Aristotle (in Book V of Physics): "Nature operates in the shortest way possible."

Or, quite simply: "The simplest explanation is the best."